THE MICROMORPHOLOGY OF YOUNGER DRYAS-AGED BLACK …

THE MICROMORPHOLOGY OF YOUNGER DRYAS-AGED BLACK MATS FROM

NEVADA, ARIZONA, TEXAS AND NEW MEXICO

by

Erin Harris-Parks

A Prepublication Manuscript Submitted to the Faculty of the

DEPARTMENT OF GEOSCIENCES

In Partial Fulfillment of the Requirements

for the Degree of

MASTER OF SCIENCE

In the Graduate College

THE UNIVERSITY OF ARIZONA

2014

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Abstract

Black mats are organic-rich sediments and soils that form in wet environments associated

with spring discharge. A micromorphological and geochemical analysis of 25 Younger Dryas-

aged (12.9-11.6 ka BP) and younger, black mats was conducted to determine their composition

and depositional environment. Samples were collected from previously documented and

radiocarbon-dated sites in Arizona, New Mexico, Texas and Nevada. Geochemically, black mats

are highly variable and can range from 0-51.0% CaCO3 and 0.4-21.8% organic matter.

Micromorphological analyses were conducted on thin sections using polarized and fluorescent

light. These analyses determined that black mats contain a variety of organic matter including

humic acids, fine (5-20μm) plant fragments, diatoms, phytoliths, and gastropods. The dominant

type of organic matter is derived from herbaceous plants, contradicting previous studies that

supported algal or charcoal sources. Differences in the micromorphological characteristics of the

samples revealed that black mats formed as three different types: organic horizons, moist soils

and diatomaceous ponded sediments. The relationship between topography and the water table

governs the formation of these different types. Black mat formation peaked in Nevada and

Arizona during and directly after the Younger Dryas due to a raised water table which provided

optimal conditions for spring discharge and heightened plant productivity.

Keywords: micromorphology, black mats, Younger Dryas, algae, desert, paleoclimate,

groundwater, thin sections, fluorescence microscopy

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Introduction

Black mats are organic-rich sediment layers and soils preserved in late Pleistocene and

early Holocene stratigraphic sequences. They are found throughout central and western North

America and even in the Atacama Desert in northern Chile (Quade et al., 1998; Haynes, 2008;

Pigati et al., 2009; Pigati et al., 2012). The term black mat has been applied to a wide variety of

soil and sediment types including Mollisols, Aqolls, algal mats, diatomites and marls (Haynes,

2008). Previous studies established that black mats form in wet environments associated with

spring discharge and elevated water tables (Quade et al., 1998; Haynes, 2008; Pigati et al., 2009).

Because black mat formation is so closely related to moist conditions, their presence in present

day arid to semi-arid environments suggests that they formed under different environmental

conditions compared to today.

Previous studies have documented dramatic increases in black mat formation associated

with the start of the Younger Dryas (YD) climatic event. Quade et al. (1998) determined that

black mat formation in southern Nevada peaked from 11,200-10,000 14C yr BP. Haynes (2008)

called black mats the “stratigraphic manifestation of the Younger Dryas.” after identifying 70

YD-aged black mats associated with archaeological sites in North America. The YD was a brief

period that lasted from 11,000-10,000 14C yr BP (12.9-11.6 ka yrs BP) and marked return to

cooler conditions following a general warming trend after the end of the last glacial maximum

(Mangerud et al., 1974; Anderson, 1997; Severinghaus et al., 1998). Greenland ice cores provide

high-resolution records of colder and dryer conditions in the North Atlantic for the YD. Some

records indicate that conditions could have been up to 15±3⁰C colder than present in the North

Atlantic (Severinghaus et al., 1998).

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However, most of North America did not experience these glacial-like conditions.

Paleoclimate studies in the southwest (SW) indicate moister conditions associated with the YD,

including speleothem growth and paleolake expansion (Broughton et al., 2000; Polyak et al.,

2004; Briggs et al., 2005). Conversely, other records indicate drying and warming trends in the

Midwest during the YD (Holliday et al., 2011; Wang et al., 2012). Additionally, the

disappearance of the Clovis culture, the earliest widespread culture to occupy all of North

America, and the extinction of an estimated 35 genera of large mammals appears to have

coincided with the onset of the YD (Haynes, 2008; Polyak et al., 2012). Therefore, the peak in

black mat formation and the onset of the YD mark major cultural and biological changes.

Adding to the significance of black mats is the theory that a 12.9 ka extraterrestrial

impact caused YD climate changes, megafaunal extinctions and the demise of the Clovis culture

(Firestone et al., 2007). Many of the proposed “impact markers” found in black mats have since

been discredited by others citing misidentification, presence in younger black mats, or non-

reproducibility (Pinter et al., 2011; Pigati et al., 2012). Firestone et al. (2007) also reported

elevated levels of charcoal, soot and carbon spherules in black mats due to widespread wildfires

caused by the impact.

Because of the debate surrounding environmental changes during the YD, and the

resulting widespread development of black mats, it is important to know the organic and mineral

composition of black mats in order to better understand how they formed. Previous studies have

identified black mats as organic rich, silty layers (Quade et al., 1998), still others have called

them algal mats (Jull et al., 1999; Haynes, 2007b) and finally at Lubbock Lake, the black mat is

in fact a white-colored diatomite (Holliday, 1985a; Haynes, 2008). In order to determine their

composition, black mat samples from Arizona, Nevada, Texas and New Mexico were collected

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and analyzed in thin section and geochemically. Here I present a thorough description of the

microscopic and geochemical contents of 25 black mat samples with a strong emphasis on the

types and forms of organic matter present. The overall goal of this study is to provide an in-depth

understanding of the composition and origins of black mats in order to relate their formation to

environmental changes associated with the YD.

Sampling Locations

Murray Springs and the San Pedro Valley, Arizona

The San Pedro Valley is a 240 km long valley, located in the Basin and Range Province of

southern Arizona on the border between the Sonoran and Chihuahuan Deserts (Pigati et al.,

2009) (Fig. 1). Many of the archaeological sites there contain evidence of Clovis mammoth

hunting (Haury, 1953; Haury et al., 1959; Hemmings and Haynes, 1969). In most of these

locations, the black mat, referred to as Stratum F1 or the Clanton Ranch Member, directly

overlies the Clovis age strata (Haynes, 2007d). It also formed contemporaneously with Stratum

F2b, the Earp Marl (Haynes, 2007c). Radiocarbon dates on the organic matter in the black mat

indicate that it formed from 10,800-9,800 14C yr B.P. (Haynes, 2007a)(Fig. 2).

Murray Springs is an archaeological site in Curry Draw, a tributary of the upper San Pedro

Valley. Excavations, which began there in June 1966, uncovered mammoth and bison remains

associated with Clovis artifacts, and a Clovis campsite (Haynes, 2007d). The black mat directly

overlies the Clovis surface and is described as an organic-rich, silty, smectitic clay with fine

angular blocky structure (Haynes, 2007c). Combined pyrolosis and thin layer chromatography

analysis of black mat samples from Murray Springs led Haynes (2007b) to conclude that the

organic matter was primarily composed of algae.

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For this study, micromorphology and bulk samples were collected from Trench 22, Area 4,

and Profile B at Murray Springs. Four samples were collected from black mat outcrops along a

35 km stretch of the upper San Pedro Valley. The basal contact of the black mat was included in

all of these samples. Four additional samples from the San Pedro Valley were provided by Jesse

Ballenger (Statistical Research Inc.).

Lubbock Lake, Texas

Lubbock Lake is located in Yellowhouse Draw, a tributary of the Brazos River, on the

Southern High Plains of northwestern Texas (Fig. 1). Lubbock Lake is a well-stratified

archaeological site, discovered in 1936, that contains evidence of continuous occupation by

Native American populations for the past 11,000 14C years (Holliday, 1985a). The Paleoindian-

aged archaeological deposits are associated with alluvial, spring, marsh, and lacustrine

sediments, and include Rancholabrean faunal remains with evidence of butchering (Stafford,

1981; Holliday, 1985a).

The black mat at Lubbock Lake is found in stratum 2A and is described as interbedded

laminations of pure diatomite, sapropelic silt, clay, and, phytoliths that contains multiple bison

kills associated with Folsom artifacts (Stafford, 1981; Johnson and Holliday, 1981; Holliday,

1985a; Johnson, 1987). The lithologic variability is interpreted to reflect alternating periods of

standing water and marsh sediments (Holliday, 1985a). Radiocarbon dates on stratum 2A show

that it formed from ~11,000 to 10,000 14C yrs BP (Holliday, 1985a; Haas et al., 1986) (Fig. 2).

For this study, a bock of sediment of the black mat including its overlying and underlying

contacts was collected from stratum 2A at Trench 65.

Blackwater Draw, New Mexico

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The Blackwater Draw Locality No. 1 archaeological site is located on the Southern High

Plains of northeast New Mexico in a basin that flows into Blackwater Draw proper, another

tributary of the Brazos River (Fig. 1). Archaeological excavations at the site uncovered Clovis,

Folsom, Agate Basin and Cody cultural artifacts, butchered animals and campsites (Haynes,

1995). It is also the site of the proposed oldest prehistoric well in North America, said to have

been dug by Clovis people ~11,500 14C yrs B.C. (Haynes, 1999).

The black mat at Blackwater Draw spans units D and E. According to Haynes (1995),

unit D is a lacustrine diatomite interbedded with eolian sands that contains Folsom artifacts and

bison bones. Unit E is composed of banded organic diatomaceous silt and sands that formed in a

marshy environment. The black mat formed from 10,800-9,800 14C yrs B.P. (Haynes, 1995)(Fig.

2). One micromorphology sample encompassing both units D and E and the lower contact with

unit C was collected. Additionally, one bulk sediment sample was collected.

Southern Nevada springs

The black mat samples from Nevada were collected from sites in the southern Great

Basin near Las Vegas (Fig. 1). Black mats form in unit E2 in this region and are characterized by

0.5-4% organic matter, are light to dark grey colored, 20-50cm thick, lack bedding, and are

dominated by silt (Quade, 1986; Quade et al., 1998). Stable carbon isotopic values indicate that

the organic content of most black mats are very low, which is typical of C3 plants such as trees,

shrubs, grasses and sedges (Quade et al., 1998). Black mat formation is documented from 11,800

to ~7200 14C yrs BP, with a peak in formation from 10,500-9500 14Cyrs BP (Quade et al., 1998).

Analysis of the faunal remains in these black mats indicates that they formed in localized spring,

marsh, and wet soil microenvironments associated with spring discharge (Narvaez, 1995; Quade

et al., 1995; Quade et al., 1998). The black mat at Gilcrease Ranch in Las Vegas Valley, Unit II,

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is described as a structureless organic-rich silt with mollusks, snails and carbonized plant

remains that follows the paleotopography of the spring. It was radiocarbon dated to 10,560±100

and 9920±90 14C yrs BP (Narvaez, 1995).

A total of eleven different black mats were sampled from this region. Black mat samples

with intact structure, collected and studied by Quade et al. (1995), Quade et al. (1998) and

Narvaez (1995) were thin sectioned. Original orientation was not preserved. Five of these

samples were too small to be thin sectioned and only bulk analyses were performed on them.

Methods

Micromorphology

Micromorphology samples must be collected as undisturbed sediment blocks in order to

preserve their original structural characteristics. Rarely, large intact peds could be pulled from

the wall and wrapped in toilet paper and packing tape. When the sediments were too friable to be

collected in this manner, plaster wraps or electrical junction boxes were used for sampling. For a

more detailed description of collection methods see Goldberg and Macphail (2003). Laboratory

preparation methods were followed according to Miller and Goldberg (2009). The samples were

oven dried at 65ºC for 48 hours. Then, the samples were impregnated using a 7:3 mixture of

unpromoted polyester resin and styrene (Advance Coatings, Westminster, MA). Additionally, 5

ml of methyl ethyl ketone peroxide was added to every liter of the mixture as a hardening agent.

The samples were left in a fume hood for seven days to allow the resin to absorb into the samples

through capillary action. Finally, the impregnated blocks were oven dried at 65ºC for 48 hours.

The samples were sent to Quality Thin Sections (Tucson, AZ) and cut into oversize (51x76cm)

thin sections. An additional 4 standard size (27x46 mm) thin sections, BM1, BM2, BM3, BM6,

were provided for this study by J. Ballenger. All thin sections were analyzed using an Olympus

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BX51 petrographic microscope equipped with transmitted polarized light and incident

fluorescent light. The thin sections were described using the terminology from Stoops (2003).

The abundance of the coarse components was determined using standard point counting

techniques.

Carbonate content

Bulk samples were collected from all sites except from the locations where J. Ballenger

provided thin section samples. The bulk samples were oven dried at 65ºC for 48 hours. The

abundance of calcium carbonate in the samples was determined using a Chittick apparatus

(Machette, 1986). This method uses the digestion of carbonate via HCl to produce CO2 gas. The

amount of gas produced from a known sample weight is then used to calculate carbonate content.

Organic matter content

The amount of organic carbon in the samples was measured using the Walkley-Black

method (Janitzky, 1986). This method uses potassium dichromate and sulfuric acid to oxidize the

organic carbon in a sample. After the reaction is complete, the volume of the unused solution is

determined using titration with ferrous sulfate. This amount is then used to calculate the

abundance of organic carbon. It is assumed that the amount of organic matter is equal to 1.724

times the amount of organic carbon present.

Site chronologies and radiocarbon dates

Black mats with well documented radiocarbon age control were specifically selected for

this study in order to better constrain the chronologic relationship between black mat formation

and the YD. A total of 63 radiocarbon dates were collected for the 20 sampling locations. An

additional 98 dates from all the published radiocarbon dates on black mats from the region

(Nevada, Arizona, southern California, Texas and New Mexico) were included for a total of 161

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black mat dates (Fig. 9). Only the San Pedro Valley sites lack radiocarbon dates. However,

several of these locations overlie Pleistocene megafauna remains, likely making them

contemporaneous with the nearby Murray Springs black mats.

Results

Black mat composition identified through micromorphology

The goal of micromorphological analysis is to determine the mineral and organic

components, and the spatial distribution of these materials through direct microscopic

observations. The following section outlines the different types of mineral and organic materials

found in the thin sections.

Coarse minerals

Quartz grains dominate the coarse mineral (>10μm) component of all the samples. The

grains are evenly distributed throughout the matrix, except in LL13-1 and CS13-1, where they

appear laminated. The most abundant grain size in all samples is silt (5-63μm) and the majority

of samples are composed exclusively of well-sorted, very fine sand (63-100μm) to silt-sized

quartz grains. Several samples contain much coarser grain sizes, including rounded, gravel-sized

(>2mm) quartz grains. Other minerals present in the samples include feldspars and micas which

never constitute more than 1% abundance and show no patterning or significant variation among

samples.

Micromass and fabric

The “micromass” component of the samples, defined as any material < 5µm, is composed

mainly of clays, very fine calcite crystals, humic acids, and, Fe-hydroxides. The majority of the

samples have a brown micromass composed of humic acids absorbed by high base-exchange

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clays, such as smectite, (Fig. 3f). (Singh, 1956; Taylor and Glick, 1998). Several samples contain

tan-colored micromass due to the presence of micrite, or microcryastalline calcite crystals.

B-fabric, or patterns made by interference colors in the micromass, also varies within the

samples (Stoops, 2003). The darkest colored samples have undifferentiated b-fabric, likely

because the dark humic acid staining masked the interference colors of the clay (Fig. 3f).

However, several samples from the San Pedro Valley display a wide variety of very distinct b-

fabrics including porostriated, granostriated, monostriated and crossstriated (Fig. 3a-d). Finally

the samples with micrite in the micromass display calcitic crystallitic b-fabric (Fig. 3e)

Coarse plant remains

Preserved plant material and plant decay products are present in all of the black mat

samples. Large plant fragments (>200 μm) with visible cell structure are absent in most of the

samples. Instead, the samples contain much smaller fragments, generally 20-50μm, in varying

abundances. These fragments are opaque to dark brown, lath shaped to rounded and only

occasionally show visible internal cell structure. In some samples these dark, fragmented plant

remains are so abundant that the entire groundmass is completely opaque and consists almost

entirely of this material. In other samples these small remains are scattered randomly throughout

the groundmass with no visible orientation. The Texas and New Mexico samples contain

elongate, dark brown plant remains all oriented parallel to the surface.

This material is part of the phytoclast organic matter classification group which is

composed of the lignin and cellulose tissue from higher plants (Tyson, 1995). Lignin is highly

resistant to decay and will remain in the groundmass long after other plant materials have

decayed (Tyson, 1995). The phytoclast group appears dark brown to black and usually has light

brown edges in transmitted white light (Fig. 3a). The opaque phytoclast subgroup appears

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opaque in thin section (Tyson, 1995) (Fig. 4b). Both of forms are present in the black mat

samples and are often indistinguishable since the thickness of the slide and the abundance of

organic matter can affect the opacity of the phytoclasts. Conversely, charcoal, which is

characterized by black color and very well defined internal cell structure is absent in most

samples and very rare in a few (Tyson, 1995; Stolt and Lindbo, 2010).

Phytoliths

Phytoliths, which are amorphous silica bodies produced by plants, are present in many of

the black mat samples (Mulholland and Rapp, 1992). Most often the phytoliths are distributed

randomly throughout the groundmass and appear to be most concentrated in the areas with the

darkest groundmass, presumably in areas with the highest concentration of decomposed plant

remains. The phytoliths found in the Lubbock Lake and Blackwater Draw samples are still

arranged in their original positions within the plant (Fig. 4c). It is impossible to determine the

three dimensional shape of the phytoliths in thin section. However, it appears as though the

pooid, rondel, elliptical, and rectangular shape classes from Twiss (1992) are represented in the

samples.

Gastropods and diatoms

Gastropod remains, often fragmented and unidentifiable, are present in five of the black

mat samples. Based on the elongate conic shape of several complete shells, these specimens

likely belong to the genus Gastrocopta (Burch, 1962) (Fig. 4d). However, observation in thin

section does not allow for species-level identification. Many fragmented shells and one complete

ostracod are also present in association with the marl deposits at Murray Springs.

Diatoms are only visible in the Lubbock Lake and Blackwater Draw samples (Fig. 4e).

They constitute 8% and 33% of the samples’ groundmass, respectively. Genus level

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identification of the diatoms was based on a study of diatom species on the High Plains by

Winsborough (1995) (Fig. 5). The most common genera represented in the samples are

Epithemia and Denticula.

Bone

Bone is present in three black mat samples. Two samples from Murray Springs contain

several very small (50-300µm) rounded bone fragments (Fig. 4f). The sample from Lubbock

Lake contains a very large bone (~5cm wide) that appears unbroken. This bone is possibly from

a bison, because bison kills are common in stratum 2A at the site (Johnson and Holliday, 1981;

Johnson, 1987).

Fluorescence microscopy

Fluorescence microscopy is often used as a compliment to micromophological analysis to

aid in the identification of organic materials and minerals that appear opaque in transmitted light

(Macphail et al., 2004; Goldberg et al., 2009; Mentzer, 2011). In particular, the nature of organic

matter under fluorescent light is indicative of characteristics such as cell type (i.e. parenchyma),

material (i.e. lignin) and decomposition state (i.e. humified) (Altetmüller and Van Villet-Lanoe,

1990). The black mat samples were observed under fluorescent light to better identify the types

of organic matter present.

Haynes (2007c) described the black mat at Murray Springs as an “algal mat” that formed as

an algal bloom on standing water. This conclusion was based on combined pyrolosis and thin

layer chromatography conducted in 1968 (Haynes, 2007b). Since these analyses, the black mat is

commonly referred to as an algal mat although no other study has ever identified, or attempted to

identify algae in a black mat (Jull et al., 1999; Haynes, 2008).

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Algal mats are primarily composed of green algae, which are easily identified using

fluorescent light, as they emit a bright yellow autofluorescence (Puttmann et al., 1994; Taylor

and Glick, 1998). Peat and coal deposits derived from algae are called alginites. These deposits

are typified by large colonies of Botryococcus algae which have visible radial cell arrangement

and branching colony structure (Taylor and Glick, 1998; Ji et al., 2010) (Fig. 6c). Laminated

algal deposits, or lamalginites, are composed of algae without recognizable structure (Taylor and

Glick, 1998). The algal remains in both lamalginites and alginites have bright yellow to orange

fluorescence in fluorescent light. Deposits containing decomposed algae, termed bituminites, are

also characterized by bright yellow to orange fluorescence that appears no different from the

appearance of well-preserved algae (Gutjahr, 1983; Puttmann et al., 1994; Taylor and Glick,

1998) (Fig. 6d). Conversely, non-fluorescing bituminite is composed of humic acids from

terrestrial organic matter (Littke, 1993; Taylor and Glick, 1998).

Observation of the black mat samples under blue fluorescent light revealed that the

phytoclasts and dark, organic-rich micromass do not fluoresce (Fig. 6). Only the samples

containing diatoms or calcitic micromass displayed a green fluorescence, however, these are

easily distinguished from the yellow color and shape of the algal colonies in an algal bloom (Fig.

6e).

Carbonate and organic content

Analyses were conducted to determine the abundance of carbonate and organic matter of 21

black mat samples (Fig. 7, Table 2). Organic matter content ranged from 0.4-21.7%. The

majority of samples contained less than 4% organic matter and only one sample contained

greater than 15% organic matter. Carbonate content was also highly variable and ranged from 0-

51.0%. However, all but two of the samples contained <20% carbonate.

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Descriptions of black mat types

Although all the black mat samples have many similarities in terms of mineral type,

abundance and organic matter, there are still significant morphological and geochemical

differences among the black mats sampled in this study. These characteristics appear to group

into three major types which were differentiated based on characteristics that provide insight into

depositional environments (Table 1).

Type I contains 9.9-21.7% organic matter and 0.5-12.9% carbonate. This type is

distinguished by angular blocky structure that often grades into fine laminations. The micromass

is composed almost entirely of finely fragmented opaque phytoclasts, which are so abundant that

the micromass appears completely opaque. In areas of the thin sections <30 μm thick, the

organic matter appears reddened along the edges, and sometimes individual cell structures are

visible (Fig. 4a). The coarse mineral component is very minor (4-11%) and silt-sized quartz

grains appear to be floating in an opaque matrix. Phytoliths are also present in these layers in

abundances of up to 2%. Often, these units are deposited on top of, or interlayered with marl

deposits.

Type II black mats have a lower organic component that ranges from 0.6-7.0% organic

matter. They are characterized by a much larger coarse and fine mineral component and the

absence of layering. These samples are divided into two groups which are differentiated by

differences in carbonate content, organic matter and micromass.

Type IIa black mats are characterized by a dark brown micromass, complex b-fabrics and

an abundant but variable coarse mineral component. Organic matter ranges from 2.5-7.0% and

carbonate content ranges from 0-3.9%. Massive and subangular blocky microstructures are most

common. Most samples are dominated by silt in 9-11% abundances. Others are poorly sorted

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with gravel to silt sized particles in abundances from 19-49%. Cracks, or slickensides with

associated monostriated or porostriated b-fabrics are present in most samples. Other b-fabrics

including crossstriated and granostriated are also present in these samples. Phytoliths are present

(1-5%) in all of the samples.

Type IIb black mats are characterized by light brown to tan colored micromass due to

higher carbonate content (2.1-52.0%). Micritic crystals are often visible in the micromass. Well

preserved opaque and non-opaqe phytoclasts are present evenly distributed in the groundmass

and are generally very finely fragmented. Organic matter content ranges from 0.6-3.3%. These

samples are also characterized by the total absence of phytoliths and the presence of gastropod

shells in all but one of the samples. The coarse component of the samples is dominated by silt

sized quartz grains with abundances of <1-10%.

Type III black mats are markedly different from the previous samples. These samples

have light grey colored massive groundmasses, very weakly speckled b-fabrics and distinct

layering where phytoclasts appear oriented parallel to the surface. Organic matter and carbonate

content is very low and ranges from 0.9-0.4% and 0-1.75% respectively. The coarse component

in these samples consists of silt grains (3-13%) with distinctly layered quartz grains. The samples

are dominated by phytoliths and diatoms that appear either well mixed or as alternating layers.

Often, phytoliths appear associated in situ with plant remains.

Discussion

Black mat composition

Opaque to nearly opaque phytoclasts were present in all samples. Dark to opaque organic

matter is formed through desiccation and oxidization when organic material is exposed to

oxygen at, or very close to the surface. This generally occurs in areas with strong seasonality and

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water table fluctuations (Tyson, 1995). The blackening of organic matter can also be caused by

waterlogged conditions (Stoops, 2003). Finally, wetting, drying, and, microbial and faunal

activity will crack and fragment the organic material into lath shapes and evenly distribute it

throughout the groundmass (Stolt and Lindbo, 2010).

The presence of repeated wetting and drying cycles during black mat formation is also

indicated by the well-developed b-fabrics in the matrix (Fig. 3a-d). Microshearing from shrink-

swell processes will cause the clay particles to align in the micromass (Kovda and Mermut,

2010). Vertic features such as these are found in many of the samples from the San Pedro valley

that are rich in clay.

The absence of large bark and leaf fragments and the abundance of very fine phytoclasts

suggests that the original plant material was derived from small herbaceous plants. Herbaceous

plants, including grasses, contain 3-10% lignin, which is the decay-resistant material that forms

phytolcasts (Tyson, 1995). The other components of herbaceous plants quickly decay into humic

acids which bind to the clay particles giving some black mat samples their dark color (Singh,

1956). The abundance of phytoliths in many of the samples also suggest an organic matter source

primarily derived from grasses (Mulholland and Rapp, 1992). Most pooid-type phytoliths,

which are common in the black mat samples, are produced in leaf epidermis of C3 grasses

(Twiss, 1992). This supports the isotopic data from Quade (1998) which attributed low δ13C

value (-22 to -27‰) of black mats to the presence of decomposed C3 plants. The major organic

input in the black mats is likely from grasses, or other herbaceous phytolith-rich plants. Finally,

the absence of ash and near-complete absence of charcoal in all of the samples does not support

the theory that the black mats formed by regionally extensive fires caused by an extraterrestrial

impact.

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Fluorescence

Previous chemical and fluorescence studies on algal deposits show that algae will fluoresce

in both preserved and highly decomposed states (Taylor and Glick, 1998). Modern evidence

suggests that coalesced algal mats are resistant to oxidization and will be preserved even under

subaerial exposure (Teichmüller 1982, Dubreuil et al., 1989). Others report that algae will decay

quickly but the gross outline of the colonies will still be visible (DeDeckker, 1988; Dubreuil et

al., 1989). Therefore, the lack of fluorescence, or other morphological indicators of algae in

black mats indicates that they did not form primarily from large colonial algal blooms. Previous

studies have demonstrated that humic rich sediments do not preserve algae (Tayor et al., 1998).

Therefore, it is possible that low abundances of algae were once present in the black mats, and

have been completely destroyed by the abundant humic acids.

The abundance of non-fluorescing organic material with visible cell structure indicate that

black mats did not form as simply from seasonal algal blooms but are instead dominated by the

input of organic material from herbaceous plants. The opaque nature of the organic matter

present in the black mats appears most similar to the degradofusinite maceral type found in coals.

Degradofusinites formed in conditions of desiccation and oxidization, causing their characteristic

non-fluorescing, poorly preserved appearance (Taylor and Glick, 1998; Goncalves et al., 2013).

Paleoenvionmental interpretation of black mat types

Type 1 Periodically Saturated Organic Horizons

In order for these organic-rich horizons to form, the production rate of organic materials

exceeded the rate of decomposition. The detrital humic plant material (<100μm) in these black

mats is very similar to those found in reed, sedge and grass marshes which are characterized by

low lignin content and highly decomposed organic matter (Cohen and Spackman, 1977; Taylor

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and Glick, 1998). These types of deposits generally form in environments with high water tables

dominated by herbaceous plants (Taylor and Glick, 1998). Bouma et al. (1990) determined that

saturated soils are characterized by layering, massive structure, and abundant diatoms. Although

layering and massive structure are present in these samples, the absence of diatoms and the

highly fragmented state of the organic matter suggests that these black mats did not form in

permanent standing water conditions.

The characteristic black color of the organic matter in these layers results from

decomposition in oxic conditions and/or microbial attack (Taylor and Glick, 1998). These layers

could be oxidized in two ways: 1) Groundwater has high oxidizing capacity. Studies show that

aquifer groundwater in Nevada and Arizona contains high amounts of dissolved oxygen

(Winograd and Robertson, 1982). 2) The desiccation of peat due to periodic drying will

promote decomposition or ripening. This allows for plant decomposition through oxidization,

faunal activity and humification resulting the formation of secondary humus peats (Babel, 1975;

Bouma et al., 1990). Alternating wet and dry periods during peat formation will create blackened

plant fragments and angular blocky structure both of which are present in the black mat samples

(Taylor and Glick, 1998; Stolt and Linbo, 2010).

The absence of diatoms, which are characteristic of prolonged permanently saturated

peats, also suggests that these samples experienced periodic desiccation (Bouma et al., 1990).

Silica can dissolve based on changes in pH, temperature and salinity (Ryves et al., 2006). The

absence of diatoms in these samples is due to either unfavorable living conditions from periodic

wetting and drying, or dissolution caused by changes in pH during periods of desiccation.

Corroded phytoliths in these thin sections also suggests silica dissolution is occurring.

19

All of the samples in this category were deposited very close to spring vents (Narvaez,

1995, Haynes, 2007c). This setting provides a readily available source of water capable of

sustaining at least seasonal periods of standing water creating highly productive plant

communities. Peat is defined by Tarnocai and Schuppli (1987) as having >17% organic carbon

(>29% organic matter). None of the type I black mats contain enough organic matter to be

considered peat possibly because of decomposition from frequent desiccation, or that they

formed over too brief a period to accumulate sufficient amount of organic matter. Regardless,

given their similarity to herbaceous peat deposits, type I black mats, which formed almost

exclusively at Murray Springs, Arizona, formed in peat-like conditions in which organic matter

accumulated in water saturated conditions with layering and low mineral input. Periodic,

possibly seasonal drying of these deposits caused the organic matter to decompose through

oxidization, and bacterial decay. This caused the organic material to appear blackened through

humification and fragment due to desiccation and breakdown. The corroded appearance of the

phytoliths suggest that these conditions likely caused silica dissolution, explaining the absence of

diatoms, which are usually characteristic of saturated sediments.

Type II Moist Soils

This type is characterized by a well-mixed groundmass and a much higher coarse mineral

component. The absence of bedding, and the even distribution of organic material and coarse

sized minerals in this type of black mat indicate that they formed in unsaturated, heavily

bioturbated conditions. Based on their micromorphological and geochemical characteristics this

type was subdivided into two groups with many samples showing intermediate characteristics

between the two. The major difference between the two groups is carbonate content, which is

determined by parent material, rather than differences in depositional environment.

20

Type IIa black mats are almost exclusively located in Arizona, and are characterized by

dark brown micromasses and vertic properties such as complex b-fabrics, slickenslides, rounded

clay and calcite anorthic clasts, and blocky structure. These are indicative of cyclic shrinking and

swelling of clays. These shrink-swell associated b-fabrics require 550 or more years to develop

indicating that these samples experienced prolonged exposure to these conditions in order to

form (Paranjape et al., 1997).

Mixing from bioturbation injects oxygen into the subsurface stimulating decomposition

of plant material, and the rapid humification of grasses (Taylor and Glick, 1998). This will

produce humic acids which bind to the smectite clays present in the San Pedro Valley giving

these samples their dark appearance.

The coarse mineral content of this type varies from well-sorted silt to poorly sorted

coarse sand-sized grains. In the coarser samples, the grain size and abundance does not change

between the black mat and the lower unit. This suggests that gravelly channels stabilized during

this period allowing plant productivity and soil forming processes to occur for a number of years.

Type IIb black mats, are largely dominated by the Nevada samples, but also include

several Arizona samples. The limestone-dominated mountains surrounding the Nevada black

mats contribute to the high carbonate contents found in this type. Type II black mats are

characterized by have lighter micromasses due to the increased abundance of carbonate, and

decreased amounts of organic matter. Humic acid staining is absent because CaCO3 inhibits the

sorption of humic acids, allowing them to be leached from these sediments (Singh, 1956).

Additionally, the presence of carbonate in sediments will reduce acidity of system, thus

promoting more rapid decay of plant materials, especially in aerobic conditions. (Taylor and

21

Glick, 1998). Therefore, the only visible organic matter present in these samples are lath shaped

phytoclasts, which have been evenly distributed throughout the sediments from bioturbation.

CaCO3 also restricts the shrink swell capacity of soils, and can mask the appearance of b-

fabrics, explaining why complex b-fabrics are absent in these samples (Pal et al., 2001; Kovda

and Mermut, 2010). The abundance of iron impregnations, nodules and hypocoatings in these

samples do however indicate that these samples formed in wet conditions.

Type II soils formed in well-aerated moist sediments. The presence of shrink swell

features and iron staining indicates that these soils experienced moist conditions that promoted

elevated levels of plant growth. The decomposed nature of the organic matter in these types is

due to biological and chemical decay processes that were able to continue, uninhibited by

saturated or anoxic conditions. This type of black mat formed as a moist soil likely close to areas

of spring discharge. They were located at slightly higher elevations, or in areas with a lower

water table than type I black mats and did not experience standing water. Instead, moist,

intensely bioturbated soils formed due to their proximity to the shallow water table below or

through slow lateral saturation due to spring seepage.

Type III Diatomaceous Ponded Sediments

The type III black mats from Lubbock Lake and Blackwater Draw have characteristics

quite distinct from the other black mats. Microlamintations of silt, phytoliths, organic matter, and

diatoms indicate that these deposits formed in very low energy, standing water environments.

Saturated conditions persisted throughout their formation as evidenced by the lack of

bioturbation and the massive structure that lacks micromorphological evidence for desiccation.

The low organic matter content can be explained by oxidization from well oxygenated ground

22

waters or formation in a shallow lacustrine environment that lacked significant plant

colonization.

The identification of diatoms in these thin sections is supported by the findings of

Winsborough (1995). At Lubbock Lake the very high abundance of E. argus, D. elegans and R.

gibberula at ~10,000 yrs BP are interpreted as a shallow muddy ponded environment, reflecting

a decrease in water level at this time (Holliday, 1995). At Blackwater Draw, the diatom

assemblage is dominated by R. gibba, and E. adnanta from ~10700-10200 and is interpreted as a

shallow pond or marsh with mud or vegetation with an estimated water depth of ≤1 meter

(Winsborough, 1995). These interpretations support the micromorphological interpretations and

indicate that these samples formed in shallow, lacustrine to marsh-like conditions of permanent,

low energy standing water. Diatoms are considered a hard-bodied form of algae. Therefore, the

white colored “black mats” on the High Plains are the only black mats to have a significant algal

component.

Facies model and regional water table inferences

The presence of black mats in arid and semi-arid regions with features related to water

presence or saturation indicate that they all formed in wet conditions. The three main types of

black mats identified can be related to interactions with local/regional topography and the water

table (Fig. 8).

The characteristics identified in type I, IIa, and IIb black mats describe the best example

of these types. In reality, several black mats can be seen grading from type I to IIa to IIb. These

types represent a continuum where either lateral or temporal changes in organic matter

productivity and mineral input can change these black mats from one type to another. Type I

black mats form in periodically saturated swamp-like environments where plant productivity is

23

very high and mineral input is very low. This type of black mat can slowly grade into type IIa,

which forms in wet soils with slightly lower plant productivity where bioturbation causes

significant mixing with the underlying sediments. As carbonate content increases, due to

limestone rich parent material, type IIa grades to type IIb.

Because all of these types occur in black mats from both Arizona and Nevada, it suggests

that the appearance of black mats is heavily dictated by the topographic relation to the water

table and to a lesser degree geographic location. The final type of black mat occurs, with great

similarity in two samples from the High Plains. These samples have evidence for permanent

standing shallow water. It is clear that these black mats formed in a system separate from type, I

and II, which lack evidence for permanent standing water conditions. The morphological

distinctions between these types suggests regional differences in effective moisture between the

High Plains and the SW deserts, which are most likely controlled by differences in larger

climatic systems.

Timing of black mat formation

Black mats are not unique to the YD. They formed in the Atacama Desert in Chile as

long as 40 thousand years ago while others formed in southern Nevada ~500ys ago (Quade et al.,

1998; Pigati et al., 2012). The micromorphological and geochemical characteristics of YD-aged

and 7,200 14Cyr-aged black mat samples in Nevada are indistinguishable, indicating that they

formed under similar environmental conditions. What makes YD-aged black mats unique is their

abundance and large geographic distribution (Fig. 9). The widespread emergence of black mats

associated with the YD indicates favorable conditions for formation occurred across a larger area

than previous or more recent times. The sharp increase in formation right at the onset of the YD

suggests that there was not a lag time between the onset of the YD and black mat formation. This

24

is possible since black mats are composed of herbaceous plants which have a much faster

response time to environmental changes than larger plants.

Black mat formation appears most abundant during the latter part of the YD and remains

elevated until ~8,500 14Cyrs B.P. (Fig. 9). Quade et al. (1998) demonstrated that modern

contamination is not a significant issue in arid settings. Therefore, it is more likely that the peak

in black mat formation from ~10,500-9,500 14Cyrs BP is due to the lasting effects of regional YD

environmental changes that appear to outlive the YD by over 1,000 years.

Other proxies for environmental conditions

Several paleoenvironmental proxies support the prevalence of wetter conditions in the

SW during the YD. Studies on speleothem growth in Arizona and New Mexico during the YD

determined that cooler moister conditions with increased winter precipitation prevailed during

the YD (Wagner et al. 2010; Polyak et al 2004; Asmerom et al., 2010). In one instance the

timing of these wetter exceed the YD by over 1,000 years, supporting the black mat radiocarbon

ages in suggesting that wetter conditions persisted long after the termination of the YD. Lake

expansion also occurred during the YD in Great Basin at Lake Bonneville and Pyramid Lake

(Broughton et al., 2000; Briggs et al., 2005).

Paleoclimatic reconstructions from various regions in the Southern High Plains lack a

universal response to the YD. Lowland areas shifted from alluvial to lacustrine environments at

the onset of the YD while upland areas experienced episodic dune accretion and soil formation

(Holliday et al., 1996; Holliday et al., 2011). These changes do suggest an overall warming trend

and decline in effective moisture that initiated during the YD and intensified during the Early

Holocene.

25

Atmospheric circulation models proposed by Wang et al. (2012), Polyak et al. (2004),

and Yu and Wright (2001) explain the seeming disparity between moister conditions in the SW

and dryer conditions in the High Plains. When the Northern hemisphere cools as it did during the

YD and the Heinrich 1, the Intertropical Convergence Zone (ITCZ) and Polar Jet Stream shift

south (Polyak et al., 2004; Wang et al., 2012). Consequently, storm tracks shift south leading to

wetter conditions in the southwestern US (Wang et al., 2012). Yu and Wright (2001)

hypothesized that arctic air was trapped by the Laurentide ice sheet during the YD, allowing

warm air from the Caribbean to penetrate into the continental interior, explaining a warming

trend in the High Plains.

Conclusions

Micromorphological analysis of 25 black mat samples indicate that all black mats contain

a variety of organic matter forms including humic acids, lignin rich plant fragments, gastropods,

phytoliths and diatoms. Contrary to previous studies, fluorescing algal colonies and charcoal are

effectively absent in all of the samples, indicating that black mats did not form as algal blooms or

from meteorite impact fires. Instead, black mats formed through the high input of herbaceous or

grassy plants, which grew in stable, moist settings. Miromorphological and geochemical analysis

of the samples showed that black mats characteristics group into three main types. Black mats

from Arizona and Nevada range from peat-like deposits to moist soils depending on their

topographic position relative to the water table. Their formation peaked in association with the

YD due to a raised water table and greater effective moisture. Conversely, black mats from the

High Plains formed as lower effective moisture lowered the water table and created shallow

ponded sediments. The differences in these systems are related to broader atmospheric

circulation changes caused by the YD. However the persistence of black mat formation beyond

26

the end of the YD suggests a lag between the termination of the YD and the localized changes it

incurred. As for the suggestions that bolide-induced climate changes affected human populations

and caused catastrophic fires across North America, black mats can no longer be used in that

argument. These sediments represent naturally occurring organic-rich deposits that formed in

response to changes in effective moisture, in a similar fashion both before, during and after the

YD.

Acknowledgements

This work was funded by the Paul S. Martin Scholarship and the Argonaut

Archaeological Research fund. I would like to thank Jesse Ballenger, Rachel Cajigas, Jennifer

Kielhofer, Amy Schott and Vance Holliday for providing field and laboratory assistance. Finally,

I would like to thank Paul Goldberg, Chris Miller, Susan Mentzer and David Killick for guidance

in thin section interpretation and access to microscopic equipment.

27

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Taylor, G. H.. Glick, D. C., 1998. Organic Petrology: A New Handbook Incorporating Some

Revised Parts of Stach's Textbook of Coal Petrology. Berlin, Gebrüder Borntraeger.

Teichmüller, M., 1982. Origin of the petrographic constitents of coal. In: Stach, E., Mackowsky,

M.T., Teichmüller, M., Taylor, G.H., Chandra, D., Teichmüller, R., Murchison D.G.,

Zierke, F, (Eds.), Stach’s Textbook of Coal Petrology, 3rd edition. Gerbruder Borntraeger,

Berlin, pp. 381-413

Twiss,P., 1992. Predicted world distribution of C3 and C4 grass phytoliths, In: Rapp, G. R.

Mulholland, S. C., (Eds.), Phytolith Systematics: Emerging Issues. New York, Plenum

Press, pp.113-128.

Tyson, R. V., 1995. Sedimentary Organic Matter: Organic Facies and Palynofacies. London,

New York, Chapman & Hall.

Wagner, J.D.M., Cole, J.E., Beck, J.W., Patchett, P.J., Henderson, G.M., and Barnett, H.R.,

2010. Moisture variability in the southwestern United States linked to abrupt glacial climate

change. Nature Geoscience 3, 110-113.

Wang, H., Stumpf, A.J., Miao, X., and Lowell, T.V., 2012. Atmospheric changes in North

America during the last deglaciation from dune-wetland records in the Midwestern United

States. Quaternary Science Reviews 58, 124-134.

Winograd, I. J., and Robertson, F. N., 1982. Deep oxygenated groundwater - anomaly or

common occurrence. Science 216. (4551), 1227-1230.

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52 (4), 333-369.

32

Figures and Tables

Figure 1. Generalized map of sample locations. Black dots indicate sampling locations. 1) Lubbock Lake. 2)

Blackwater Draw. 3) BM 3. 4) Murray Springs, BM6, BM5, SP13-1, SP13-2, SP14-3, SP14-4. 5) Cactus Springs. 6)

Corn Creek Flat. 7) Gilcrease Ranch. 8) Browns Spring, Stump Spring. 9) Sandy Valley.

33

Figure 2. Late Pleistocene and Early Holocene stratigraphy from the black mat sampling locations based on Haynes,

1995; Holliday, 1995; Quade et al., 1995; Quade et al., 1998; Haynes, 2007c.

34

Figure 3. (A) Crossstriated b-fabric, XPL(cross polarized light) in SP13-1. (B) Monostriated b-fabric, XPL in BM2.

(C) Porostriated b-fabric, XPL in BM6. (D) Granostriated b-fabric, XPL in BM2 (E) Calcitic crystallitc micromass,

XPL in Pah Carb 20. (F) Humic acid-stained micromass PPl (plane polarized light) in BM3.

35

Figure 4. (A) Phytoclast with visible and connected cell structure, PPL in MS13-1a. (B) Opaque phytoclast, PPL in

BM1. (C) Aligned rows of phytoliths (p), PPL in LL13-1. (D) Whole gastropod shell (g), XPL in MS13-2B. (E)

randomly oriented phytolcasts that appear as opaque black fragments in a calcite rich matrix, PPL in Pah Carb 26.

(F) Bone fragment (b), PPL in MS13-2B.

Figure 5. (A) Epithemia argus, (B&C) Epithemia adnata from Winsborough (1995). (D) A species of Epithemia

from Lubbock Lake, PPL. (E) A species of Epithemia from Blackwater Draw, PPL. (F&G) Denticula elegans from

Winsborough (1995). (H&I) A species of Denticula from Lubbock Lake, PPL.

36

Figure 6. (A) Visible cell structure in blue fluorescent light in Pah Carb 20. (B) The same phytoclast appears opaque

in transmitted white light, Pah Carb 20. (C) Well preserved Botryococcus colonies from Mendonca et al. (2010),

fluorescent light. (D) Filamentous lamalginite with telalgnite (round) in oil shale, blue fluorescent light from

Petersen et al. (2006). (E) Layered black mat in green fluorescing marl, blue fluorescent light in MS13-2B. (F)

Layered organic matter in marl, PPL in MS13-2B. (G&H) Opaque organic matter lacking fluorescence. The resin

has a dull green fluorescence, and the yellow fluorescing objects are phytoliths, blue fluorescent light in MS13-1. (I)

Opaque organic matter with cell structure, blue fluorescent light in SVF-87-9. (J) Modern root with bright yellow-

green fluorescence, blue fluorescent light in MS13-2B.

37

Figure 7. Percent calcium carbonate versus percent organic matter among types I-III black mats. The two samples

from Nevada that plot near the type III black mats do not fit into a category based on their geochemical and

micromorphological characteristics.

Figure 8. Facies model for black mat formation. The model demonstrates how the relationship between groundwater

and topography can affect the morphological and chemical characteristics of black mats. Note that this exact

progression is not seen in any region where black mats where sampled. Type III is geographically restricted to the

High Planes while type I and II area seen grading into each other in both Arizona and Nevada.

38

Figure 9. All radiocarbon dated black mat samples in the areas near the sampling locations. Filled circles indicate

radiocarbon dates taken from the sample locality, and in most cases the 14C sample was collected from the same

outcrop as the micromorphology samples. Open circles indicate dates from nearby localities. Dating Sources:

Haynes, 1967; Haas and Holliday, 1986; Quade, 1986; Haynes, 1995; Narvaez, 1995; Quade et al., 1995; Quade et

al., 1998; Jull et al., 1999; Huckleberry et al., 2001; Haynes, 2007a; Haynes, 2008; Hatte et al., 2010; Pigati et al.,

2011; Pigati et al., 2012.

39

Table 1

Descriptions of each black mat type with a photo of the characteristic mircromorphological features, a list of

samples for each type, and their interpreted depositional environment.

40

*From Holliday 1985b, + thin sections from J. Ballenger #Bulk sediment sample only, no thin section, ^ poorly preserved thin section sample, no description

included in Table 2.

Table 2

Basic geographic, geochemical and chronologic characteristics of the black mat samples.

Sample name Thin

section

ID

Location %

o.m.

%CaCO3 Type Age 14Cyrs/relative Dating source

Nevada svf87-9 1 Sandy valley 4.0 0 IIa 9620±110 Quade et al. 1998

Gilcrease1 2 Gilcrease Ranch 1.0 1.2 - 10560±100, 9920±90 Narvaez 1995

CS81 carb 6 3 Corn Creek Flat 3.3 51.0 IIb 9220±180 Quade et al. 1995

Pah Carb 20 4 Browns Springs 2.2 8.2 IIb 7230±100 Quade et al. 1998

Cac.Spr. Mol 31 5 Cactus Springs 1.5 19.2 IIb <9460±60 Quade et al. 1998

Pah Carb 26 27 Stump Spring 2.1 3.1 IIb 10090±100 Quade et al. 1998

Cac.Spr Carb 7# - Cactus Springs 0.9 0 - 10060±200 Quade et al. 1998

Cac.Spr.Carb 6# - Cactus Springs 1.8 10.2 IIb 10410±110 Quade et al. 1995

Gilcrease 16 # - Gilcrease Ranch 10.2 7.6 I 10560±100, 9920±90 Narvaez 1995

Gilcrease 22# - Gilcrease Ranch 0.6 9.5 IIb 10560±100, 9920±90 Narvaez 1995

CSC89-7# - Corn Creek Flat 2.4 16.1 IIb 10,200±130 Quade et al. 1998

Arizona MS13-1A&B 7, 10 Murray Springs Area4 9.9 1.5 I 10760±100,10260±140 Haynes 2007a

MS13-2A 13 Murray Springs Profile B 12.9 12.5 I 10325±45, 10630±60,

9820±45

Jull et al. 1999

MS13-2B1&2 14,15 Murray Springs Profile B 21.7 11.4 I 10325±45, 10630±60,

9820±45

Jull et al. 1999

MS14-4 23 Murray Springs TR 22 3.1 0.4 IIa 10260±430, 9410±160, 9240±270 Haynes 2007a

SP13-1B 17^ San Pedro Valley 7.0 3.9 IIa Directly overlies mammoth teeth J. Ballenger

SP13-2A&B 18, 19 San Pedro Valley 2.5 1.1 IIa Directly overlies mammoth teeth J. Ballenger

SP14-3 24 San Pedro Valley 10.4 0.5 I - -

SP14-4 26 San Pedro Valley 1.5 3.5 IIab Overlies Pleistocene horse tooth This study

BM1+ BM1 San Pedro Valley - - IIa - -

BM2+ BM2 San Pedro Valley - - I - -

BM3+ BM3 San Pedro Valley - - IIa - -

BM6+ BM6 San Pedro Valley - - IIab Overlies Pleistocene horse bone J. Ballenger

New

Mexico

CS-14-1 20 Blackwater Draw 0.9 1.8 III 10740-9860 Haynes 1995

Texas LL13-1 21,22 Lubbock Lake 0.4* 0* III 10780-9905 Haas et al. 1986

41

Appendix

Table A1

Micromorphological descriptions of black mat thin sections. For an overview of the terminology used see Stoops (2003). Sample

ID

Coarse Fraction Microstructure Voids c/f dist Micromasss and b-

fabric

Pedofeatures Organic matter

SVF87-9 Very well sorted quartz

grains (5%)

Weak subangular

blocky

Channels,

vughs, planes

Open

porphyric

Calcitic speckled Illuvial clay, iron

staining and

hypocoatings

Dark stains, large

phytoclasts and

phytoliths

Gilcrease

1

Well sorted silt sized

quartz grains (37%),

feldspar and mica rare

Granular Complex

packing,

channel

Chitonic Brown, weak

granostriated

Polymorphic

o.m.*

CS81Carb

6

Silt sized quartz grains

rare (<1%)

Granular to

subangular blocky

Compound

packing,

channels,

vughs, planes

Fine monic Calcitic,(dull tan),

very weakly

speckled

Transported

micrite grains,

rounded

Lath shaped

phytoclasts

dispersed through

matrix

Pah

Carb20


quartz grains (6%), mica

rare

Strong subangular

blocky

Channels and

intrapedal

planes

Open

porphyric

Micrite with visible

crystals. Tan to

brown with patchy

o.m. stains

Iron impregnations

and hypocoatings,

Large phytoclasts

with visible

structure,

fragmented

gastropod shells

Calc Spr

Mol31



mica rare

Granular to

massive with

channels

Complex

packing,

channel

Single to

fine equal

enaulic

Inhomogeneous,

calcitic,

Rounded,

transported,

micrite grains

Gastropod shells,

small phytoclasts,

also stains

Pahcarb26 Very well sorted silt

sized quartz (9%)

Massive/channel Channel and

cracks

Open

porphyric

Inhomogeneous,

calcitic, brown-tan,

weak speckled

Rounded calcite

clasts, Iron

impregnations

Large phytoclasts,

rare fragmented

shell

MS13-1A Very well sorted, silt

and fine sand quartz

(4%)

Angular blocky Channels,

intrapedal

planes

Open

porphyric

Opaque black, very

weak speckled

Needle calcite in

voids, mite feces,

iron impregnations

Opaque

phytoclasts,

phytoliths:~1%

MS13-1B Very well sorted, silt

and fine sand quartz

(5%)

Angular blocky Channels,

intrapedal

planes

Open

porphyric

Opaque black, very

weak speckled

Needle calcite in

voids, mite feces

Opaque

phytoclasts,

phytoliths

MS13-

2B1

Well sorted silt to fine

sand sized quartz grains

(8%).

Massive layered,

channel

Channel,

planes

Open

porphyric

Layered, black

organic rich and tan,

micrite

Root channels Fragmented

shells, bone,

opaque

phytoclasts

42

MS13-

2B2

Moderately sorted silt to

medium sand sized

quartz grains (2-7%)

varies in different

layers.

Sub-angular

blocky to massive

layered

Channel,

intrapedal

planes

Open

porphyric

Layered, black

organic rich and tan,

micrite

Root channels with

needle calcite

Fragmented

shells, bone,

opaque

phytoclasts

MS14-4 Very poorly sorted

rounded to subrounded,

gravel to medium sand

sized quartz grains

(49%)

Crumb Complex

packing

Double

spaced

equal

enaulic

Dark brown-brown ,

grano and parallel

striated, speckled in

layered area

Modern root and

mite feces

Phytoliths rare,

stains

SP13-1B Poorly sorted, rounded,

fine gravel to silt sized


feldspar (2%)

Massive with

channels

Channel,

vughs

Open

porphyric

Patchy dark brown to

grey, weak poro,

grano and

crossstriated

Phytoliths: 2-5%,

polymorphic o.m.,

dark stains

SP13-2A Poorly sorted

subrounded coarse sand

to silt sized quartz

grains (19%) feldspar

(1%)

Subangular blocky Planes,

cracks, some

channels

Double

spaced

porphyric

Brown, weak

speckled, some mono

and porostriated

Phytoliths rare,

small phytoclasts

SP13-2B Moderately sorted, fine

gravel to silt sized

rounded quartz grains

(14%), mica (1%)

Subangular blocky

to angular blocky

Planes,

cracks,

complex

packing

Double to

open

porphyric

Brown-light brown,

grano, poro and

crossstriated and

speckled

Iron staining, root

bioturbation

Phytoliths rare,

small phytoclasts

SP14-3 Poor- moderately sorted,

medium sand to silt

sized particles,

abundance increases (6-

11%)upwards, graded

bedding

Highly separated

subangular blocky,

some massive

Planar,

vughs,

channels

Double

spaced

porphyric

Brown to black,

Weakly speckled

bioturbation Phytoliths:2%,

opaque

phytoclasts,

polymorphic o.m.

SP14-4 Well sorted, fine sand to

silt sized quartz grains

(11%), mica grains rare

Well-developed

subangular blocky

to columnar

Planar,

channels

Double

spaced

porphyric

Light-brown grano,

poro, crossstriated,

and weak speckled

Iron stains,

burrows and mite

feces

Phytoclasts,

polymorphic o.m.

BM1 Poor-moderate sorted

coarse sand to silt sized


feldspar and mica rare

Intergrain

microaggregate to

subangular blocky

Complex

packing,

channels

Chitonic

and single

to double

fine

enaulic

Dark brown-black

o.m. aggregates

Thick Needle

calcite, clay clasts

Phytoliths:2%,

polymorphic o.m.

43

BM2 Poorly sorted very

coarse sand to silt sized


mica and feldspar rare

Massive with

cracks

Channels,

cracks

(slickensides)

Double

spaced

porphyric

Light-Dark brown,

o.m. stains. Strong

monostriated and

speckled, some

grano/porostriated

Clay clasts Phytoliths .5%,

polymorphic o.m.,

large phytoclasts

with cell

structure rare

BM3 Well sorted, dominantly

silt sized with medium

and fine sand sized

quartz grains (13%)

Well-developed

subangular blocky

with highly

separated peds

Planes,

cracks

Double

spaced

porphyric

Homogeneous dark

brown, weak

speckled

Clay clasts, iron

staining

Phytoliths 1%,

large phytoclasts

with cell structure

and polymorphic

o.m. rare

BM6 Very well sorted fine

sand to silt sized quartz

grains (9%), rounded

micrite fragments rare

Well developed-

moderate angular

blocky, moderate

to well separated

Cracks

(slickensides)

, planes

channels,

Double

spaced

porphyric

Brown-light brown,

Strong mono, grano,

crossstriated and

speckled

Micrite clasts, iron

staining

phytoclasts

CS-14-1 Well sorted fine sand to

silt sized quartz grains

(13%), some layering

Massive with

channels

Channels Massive

layered

Light grey to tan,

very weak speckled,

monostriated rare

Root channels Phytoliths,

diatoms,

phytoclasts all

abundant

LL13-1 Silt sized quartz grains

(3%), distinct layering

Massive with

channels

Channels,

complex

packing

Massive

layered

Light grey, weak

speckled

Root channels,

Phosphate staining

Phytoliths,

diatoms,

phytoclasts all

abundant

*organic matter

44

Appendix

Table A2

Black mat radiocarbon dates used in Fig. 9.

Location Site/ Unit Name Sample ID 14C age Source

Arizona Chapo Ranch, AZ AA-53781 9580±170 Haynes 2008

Chapo Ranch AA-52885 10516±75 Haynes 2008

Chapo Ranch AA-52886 10696±57 Haynes 2008

Horsethief Draw, Earp Marl SPV02-HD1-7 8950±40 Pigati et al. 2009

Horsethief Draw, Earp Marl SPV07-HD1-7 9010±40 Pigati et al. 2009

Murray Springs, Profile B AA-26211 10325±45 Jull et al. 1999



Murray Springs, Area 1 16MS67A 8900±400 Haynes 2007a

Murray Springs, Area 1 17ms67c 10250±170 Haynes 2007a

Murray Springs, Area 1 15ms67b 10360±90 Haynes 2007a

Murray Springs, Trench 13N 51A70 8600±240 Haynes 2007a












Murray Springs, North of RR 2A70 10680±140 Haynes 2007a



Murray Springs, Area 8 28A70 10260±430 Haynes 2007a



Murray Springs, Area 4 H-7 10760±100 Haynes 2007a

Murray Springs, Area 4 4 29 10260±140 Haynes 2007a

Murray Springs, Trench 28 23-21 9790±160 Haynes 2007a

Murray Springs, Trench 28 23-21 9850±160 Haynes 2007a

Murray Springs, Trench 13N 52 8870±140 Haynes 2007a





45








Murray Springs, Trench 18 28 10260±430 Haynes 2007a



Naco site AZ 3a 9250±30 Haynes 2008

Red Peak Valley, AZ 4a 9660±150 Haynes 2008

Seff Locality, Earp Marl SPV05-SL-1 10570± 60 Pigati et al. 2009

Wilcox Playa AZ Stratum C 8910±280 Haynes 2008

California Valley Wells, Unit E2b VW7-19 9650±100 Pigati et al. 2011

Valley Wells, Unit E2b VW2-7 9990±80 Pigati et al. 2011

Valley Wells, Unit E2b

VW1-3 10080±100 Pigati et al. 2011


VW26-69 10150±80 Pigati et al. 2011


VWF87-1b 10250±320 Quade et al. 1995


VW23-67 10320±80 Pigati et al. 2011


VW23-67 10470±80 Pigati et al. 2011


VW1-3 10670±120 Pigati et al. 2011


VW7-19 10780±80 Pigati et al. 2011


VW24-68 11190±240 Pigati et al. 2011


VWC87-3b 11600±240 Quade et al. 1998

Dove Spring MOJ07-105-DS 9950±50 Pigati et al. 2012



Nevada Browns Springs PVcarb22b 7230±100 Quade et al. 1998

Browns Springs PVCarb.21b 8210±210 Quade et al. 1998

Cactus Springs Cac.Spr.Mol 31 <9460±60 Quade 1989

Cactus Springs Cac.Spr.Carb 7 10,060±200 Quade et al. 1998

Cactus Springs Cac.spr.Carb 6 10,410±110 Quade et al. 1995

Cactus Springs Cac SprCarb-2b 9460±60 Quade and Pratt 1989

Cactus Springs Cac SprCarb-2b 9680±100 Quade and Pratt 1989

Corn Creek Flat CS81 Carb 6 9220±180 Quade et al. 1995

Corn Creek Flat CSC87-7 10200±130 Quade et al. 1998

Corn Creek Flat CSC87-5b 6340±260 Quade et al. 1998


Corn Creek Flat CSCarb.-11a 6220±250 Quade 1986

Corn Creek Flat CS81Carb11b 8640±150 Quade 1986

Corn Creek Flat CS81 carb. 3a 10090±160 Quade 1986

Corn Creek Flat CS81carb.3b 10140±130 Quade et al. 1995

46

Corn Creek Flat 75a 10200±350 Haynes 1967

Corn Creek Flat CS81Carb 13b 10220±210 Quade et al. 1998




Corn Creek Flat CSWood1 11540±50 Quade et al. 1998





Corn Creek Flat CSC27b 12100±60 Quade et al. 1998

Corn Creek Flat CSCarb.30b 12180±110 Quade et al. 1998


Corn Creek Flat CSCarb.28a 12410±60 Quade et al. 1998


Corn Creek Flat CSC-29b 12810±60 Quade et al. 1998

Gilcrease Ranch, Unit II Beta-78244 10560±100 De Narvaez 1995

Gilcrease Ranch, Unit II Beta-78243 9920±90 De Narvaez 1995

Gilcrease Ranch 62a 9200±250 Haynes 1967

Gilcrease Ranch 63a 9920±150 Haynes 1967

Gilcrease Ranch 19b 10810±460 Haynes 2008

Hidden Valley PVCarb.-47b 7920±160 Quade et al. 1998

Hidden Valley PVCarb.37b 8480±160 Quade et al. 1998



Hidden Valley PVCarb-41b 8600±130 Quade et al. 1998










N. Coyote Springs NCySC-4b 6670±50 Quade et al. 1998








47

S. Coyote Springs SCySCarb-1b 9970±90 Quade et al. 1995

Sandy valley SVF87-9 9620±110 Quade et al. 1998

Sandy Valley SAVCarb-1b 11020±140 Quade et al. 1995

Stump Spring Pah Carb 26 10090±100 Quade et al. 1998

Stump Spring Pvcarb.-15a 8570±170 Quade et al. 1995

Stump Spring PVCarb.-14b 9060±60 Quade et al. 1998

Stump Spring PVCarb.-27b 9440±50 Quade et al. 1998

Stump Spring PVCarb-13b 9650±70 Quade et al. 1998




Stump Spring PVCarb-8a 10450±150 Quade et al. 1998

Sunshine locality NV 69781 8560±100 Huckleberry et al. 2001






New Mexico Blackwater Draw AA-1361 10640±110 Haynes 1995

Blackwater Draw AA-1363 10160±120 Haynes 1995



Blackwater Draw A-4702 9870±320 Haynes 1995

Blackwater Draw A-1372 10250±200 Haynes 1995



Pounds Playa NM AA-66775L 9620±90 Haynes 2008

Pounds Playa NM AA-66775H 11700±70 Haynes 2008

Texas Aubrey site, TX, Strata E&G 10085±80 Haynes 2008

Aubrey site, TX Strata E&G 10940±80 Haynes 2008

Lubbock Lake SI-3203 10015±75 Haas and Holliday 1986


Lubbock Lake LL65-2A 10780±300 Hatte et al. 2010

Lubbock Lake SMU-251 10060±70 Haas and Holliday 1986



Lubbock Lake SMU-285 10530±90 Haas and Holliday 1986

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